class ActiveSupport::DescendantsTracker::WeakSet

:nodoc:
So we can simply use WeakMap as a ‘Set`.
On MRI `ObjectSpace::WeakMap` keys are weak references.

def <<(object) 

def <<(object)
  self[object] = true
end

def <<(object) 

def <<(object)
  self[object] = true
end

def [](object) 

def [](object)
  @map.key?(object.object_id)
end

def []=(object, _present) 

def []=(object, _present)
  @map[object.object_id] = object
end

def initialize

:nodoc:
have the same WeakMap semantic than Truffle so we future proof this as much as possible.
JRuby for now doesn't have Class#descendant, but when it will, it will likely

So we use `object_id` as a key and the actual object as a value.
On TruffleRuby `ObjectSpace::WeakMap` keys are strong references. 
def initialize
  @map = ObjectSpace::WeakMap.new
end

def to_a 

def to_a
  @map.values
end